Synthesis and Magnetic Properties of Conventional and Microwave Calcined Strontium Hexaferrite Powder

doi:10.4236/msa.2011.26087

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Materials Sciences and Applicatio ns, 2011, 2, 638-642

doi:10.4236/msa.2011.26087 Published Online June 2011 (http://www.SciRP.org/journal/msa)

Synthesis and Magnetic Properties of Conventional

and Microwave Calcined Strontium Hexaferrite

Powder

Kanagesan Samikannu1, Jesurani Sinnappan1,2, Sivakumar Mannarswamy1, Thirupathi Cinnasamy1,

Kalaivani Thirunavukarasu1

1Center for Material Science and Nano Devices Department of Physics, SRM University, Kattankulathur, India; 2Department of

Physics, Jeyaraj Annapackium College for Women, Periyakulam, India.

Email: [email protected]

Received December 24th, 2010; revised March 21st, 2011; accepted May 17th, 2011.

ABSTRACT

Powders of strontium hexaferrite (SrFe12O19-SrF) have been prepared by the sol-gel process. The prepared precursor

was calcined in two different calcination techniques, using conventional furnace and microwave furnace. Thermal

analysis studies showed exothermic and endothermic reaction peak at room temperature to 1200˚C. An investigation of

SrFe12O19 crystalline powder from the structural and magnetic aspect is performed using X-ray diffraction (XRD), high

resolution scanning electron microscopy (HR-SEM) and vibrating sample magnetometer (VSM). The average particle

diagonal size of SrFe12O19 powder was 80 - 100 nm in conventional and 40 - 70 nm in microwave calcinations respec-

tively. XRD result showed the formation of SrFe12O19 of the sample calcined at 900˚C with Fe/Sr: D-Fructose ratio = 12.

Keywords: Sol-Gel, Strontium Hexaferrite, D-Fructose, Magnetization

1. Introduction

The M-type Strontium Hexaferrite-SrFe12O19 (SrF) is a

hard magnetic material due to its high coercivity.

SrFe12O19 crystallize with 64 ions per unit cell on 11 dif-

ferent symmetry sites, the magnetic structure given by

Gorter model, 24 Fe3+ atoms are arranged over five dis-

tinct sites: three octahedral sites and two tetrahedral sites.

These five sites are coupled via ferromagnetic superex-

change interaction through O2− ions [1]. Recently, Wang

et al., reported a correlation between magnetic properties

and particles morphology of SrFe12O19 [2]. By control-

ling the microstructure, morphology and chemical com-

position and particle size, the magnetic properties of the

material can be improved. Ferrites, typically spinel ferrite

and magnetoplumbite ferrite, can be used as recording

materials, microwave devices, humidity sensors, pig-

ments etc. Compared with spinel ferrite, magnetoplum-

bite ferrites, strontium ferrite have attracted more scien-

tific research in recent years due to their high uniaxial

magnetic anisotropy, high saturation magnetization and

high coercivity [3,4]. In order to get homogeneous ferrite,

several techniques have been used in the preparation of

Sr hexaferrite, such as the chemical co precipitation [5],

hydrothermal [6], sol-gel [7,8], micro emulsion [9], and

citrate precursor [10] have been developed. Therefore,

the preparation of SrFe12O19 having high purity, ultrafine

size, good dispersion and excellent magnetism has been

the focus of recent research [7,11]. The growing interest

during the past decade is essentially due to the fact that

microwave heat treatment could influence the micro-

structure can improve the material properties. Conven-

tional furnace heating samples by the surface heating

mechanism and depending on the rate of heating, a large

thermal gradient from the surface to the centre can be

generated within a sample, particularly with materials

heaving a poor thermal conductivity. Microwave heating

would be promising because it is a self-heating process

of absorbing the electromagnetic energy. As a result of

the internal and volumetric heating at high heating rate

may lead to reduction in manufacturing cost on account

of energy savings, shorter processing times and improved

product uniformity and yields, limited grain growth

throughout the ceramic body. It is well known that the

heating rate and thermal efficiency of the microwave

heating is higher than those of conventional method [12,

13]. In this paper, we report the preparation of SrFe12O19

Synthesis and Magnetic Properties of Conventional and Microwave Calcined Strontium Hexaferrite Powder639

by sol-gel process and important hysteresis parameters of

the powders with influence of convention and microwave

calcinations.

2. Experimental

The powder was prepared by sol-gel technique using

D-Fructose as the fuel. Analytical grade Ba(NO3)2,

Fe(NO3)3·9H2O and D-Fructose were used as starting

materials. Nitrate and fuel ratio is 1:1. Stoichiometric

amount of metal nitrates and fuel were taken, dissolved

in distilled water and stirred by magnetic stirrer for 2 h to

get a solution. Sol was heated at 80˚C with stirring con-

tinuously, finally it changed in to sticky liquid gel and it

was preheated at 130˚C in a hot air oven for two days to

get precursor. The precursor was calcined (Conventional

and microwave) at 900˚C, to get crystalline barium

hexaferrite powder.

2.1. Characterization

Thermo gravimetric analysis of the mixture composed of

barium nitrate, iron nitrate and D-Fructose (precursor)

were carried out between 28˚C and 1200˚C on NETZSCH

STA 409 C/CD in the static air atmosphere at the heating

rate of 10˚C per minute. The DTA analyses of same

mixtures were also carried out on the same instrument at

same condition. The crystalline phases were identified by

means of X-ray diffraction (XRD) measurements (PANa-

lytical X’pert pro) CuKα radiation at 45 kV and 40 mA

(λ = 0.15406 nm) in a wide range of 2θ (10˚ < 2θ < 80˚).

The surface morphology and size of the ferrite particles

were studied by using FEI Quanta FEG 200-High resolu-

tion scanning electron microscope (HR-SEM) and Mag-

netization measurements at room temperature were car-

ried out on Lakeshore Vibrating Sample Magnetometer

(VSM) at a maximum applied field of 15,000 Gauss at

room temperature.

3. Results and Discussion

The thermogram of the precursor of barium hexaferrite

derived by mixing of barium nitrate, ferric nitrate and

D-Fructose as shown in Figure 1. TGA shows the initial

weight loss from 28˚C to 185˚C due to the loss of ab-

sorbed water [14]. The subsequent loss up to 400˚C is

associated mainly to the decomposition of the D-fructose.

In order to verify this, separate TGA was undertaken for

D-fructose, thermogram is shown in Figure 2. It shows

the major weight loss between 200˚C and 400˚C thus

supporting our assignment. Therefore, D-Fructose pro-

vides self heat to promote the reaction and to reduce the

crystallization temperature of the hexaferrite. The stage

of decomposition between 400˚C and 775˚C is due to

decomposition of nitrates and starting formation of

hexaferrite. There is no considerable weight loss above

Figure 1. TG-DTA curves for the precursor.

Figure 2. TG-DTA curves for D-fructose.

900˚C, confirming the formation of the stable Strontium

hexaferrite this analysis, therefore illustrates the optimum

calcinations temperature for Strontium hexaferrite is

around 900˚C.

The sequences of reaction taking place is shown in the

following steps

200C - 500C

PrecursorFe O+SrO



250C - 750C

23 24

Fe OSrOSrFe O 



above 750 C

232412 19

5Fe OSrFe OSrFeO 



Figure 3 shows the XRD patterns of the powders con-

ventionally calcined at temperatures 500˚C, 750˚C and

900˚C for 3 h in air, respectively. The precursor is cal-

cined at 500˚C; the powders can be described as Fe2O3

and SrO and then the phase of SrFe2O4 and hexagonal

SrFe12O19 can been detected for samples calcined at

750˚C. Clear diffraction peak of SrFe12O19 can be ob-

tained at 900˚C, which coincides with the JCPDS file

Synthesis and Magnetic Properties of Conventional and Microwave Calcined Strontium Hexaferrite Powder

640

number: 84-1531. Calcination temperature and interme-

diate Fe2O3 plays an important role in the formation of

Strontium Hexaferrite. The phase development micro-

wave calcined powder at different temperatures, 500˚C,

750˚C and 900˚C for 10 minutes, the peaks correspond-

ing to the standard diffraction pattern of SrO, SrFe2O4

and SrFe12O19 and is shown in Figure 4.

In order to visualize the conventional calcined powder

are in elongated hexagonal like structure, diagonal size

vary in the range of 80 - 100 nm and it is not well de-

fined shape (Figure 5). The HR-SEM micrograph for the

microwave calcined powder at 900˚C for 10 minutes is

shown in Figure 6. The particles are hexagonal platelets

and well crystalline strontium ferrite, diameters are in the

range of 40 nm to 70 nm. This type of shape is usually

observed for BaFe12O19 or SrFe12O19 obtained by sol-gel

process [4,15]. The morphology of the microwave cal-

cined powder samples reveal smaller particles compared

Figure 3. XRD patterns of the powders calcined at different

temperatures: (a) 500˚C, (b) 750˚C, and (c) 900˚C for 3 h.

Figure 4. XRD patterns of the powders microwave calcined

at different temperatures: (a) 500˚C, (b) 750˚C, and (c)

900˚C for 10 minutes.

Figure 5. HR-SEM image of conventionally calcined pow-

der at 900˚C for 3 h.

Figure 6. HR-SEM image of microwave calcined powder at

900˚C for 10 minutes.

to the conventionally calcined powder. Hard magnetic

materials with hexagonal structure is mainly due to the

microwave energy coupled through polarization, elec-

tronic and ionic conductivity loss therefore a smaller

particle size resulted from the enhanced diffusion and

accelerated densification [16].

Figure 7 shows the magnetization versus applied field

for conventional and microwave treated samples at room

temperature. The reduction in Ms in microwave calcined

powder can be attributed to the decrease in the size of the

particles. The observed value of saturation magnetization

47 A·m2/kg for the sample conventionally calcinated at

900˚C are far from the theoretical Ms value of 74.3

Synthesis and Magnetic Properties of Conventional and Microwave Calcined Strontium Hexaferrite Powder641

Figure 7. Magnetization curve of SrFe12O19 (a) powder

conventionally calcined at 900˚C for 3 h; (b) powder mi-

crowave calcined at 900˚C for 10 minutes.

A.m2/kg and the coercivity 6,709 Gauss very close to the

theoretical Hc. Observed magnetization values are close

to those observed in other methods of preparation (50 -

60 Am2/kg) [5,6,10,17]. The value of Mr (26.58 Am2/kg)

is approximately 59% of Ms it has maximum coercivity

of 6,708 Gauss for microwave calcined powder is lower

than those of the literature and of the theoretical limit

(7,500 Gauss) [18]. The samples calcined conventionally

and microwave at 900˚C shows smooth hysteresis loop,

which confirms the formation of pure strontium hexafer-

rite [18,19].

4. Conclusions

The effective influence of conventional and microwave

on the structure and magnetic properties crystalline

SrFe12O19 are discussed. The samples were subjected to

two different heat treatments. From the analysis of vari-

ous characterization techniques such as XRD, HR-SEM

and VSM, we observe that the structure remained intact

with different heating treatment process. The possibility

of lowering the synthesis temperature and get a pure SrF

powder, microwaves allows the reduction of particle size

in the hexaferrite. The external diameters of the obtained

different method of calcined SrFe12O19 particles range

between 40 to 100 nm. The results indicate that calcina-

tions method has significant effect on the saturation

magnetization (Ms). These magnetic materials can poten-

tially be used in micro/nano electronic devices, gas sen-

sors and catalysts.

5. Acknowledgements

The authors thank SRM University for providing the

facilities available in Nanotechnology center.

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